How do I combine data from multiple log files? How do I tweak my "queries" without scanning all of the data again? You get this kinds of things (joins, indexes, etc) for free from RDBMS. If I'm analyzing log files I have to write my own code to do it.

I'm not saying RDBMS is the best solution for everything and neither is OP. But it's appropriate when you don't necessarily know every way you want to access your data up front.

Many of the more advanced log analysis tools actually parse the logs and put them in a SQL database.